1. Field of the Invention
The present invention relates to a surface acoustic wave device, more particularly, to a surface acoustic wave device used as a band-pass filter, etc., of mobile radio equipment, and the like.
2. Description of the Related Art
Recently, surface acoustic wave devices (which may be abbreviated hereinafter as SAW devices) have been developed and provided for RF circuit integrations of communication equipment such as mobile telephone transceivers, cable TV repeaters and converters, and the like.
The SAW device comprises a piezoelectric substrate and inter-digital transducers (which may be abbreviated hereinafter as IDTs) disposed thereon for converting a voltage into a surface acoustic wave, or a surface acoustic wave into a voltage. Note, the SAW device including IDTs converts a high-frequency voltage into a surface acoustic wave having a wavelength of about 10.sup.-5 times. The wave is propagated on the surface of the piezoelectric substrate and again converted into a voltage by the IDTs.
The SAW device provides a filter function for selecting frequencies by the above described double conversion and depending on the shapes of the IDTs. Note, the SAW device can cause the surface acoustic waves to resonate by using a plurality of the IDTs, or reflectors for reflecting surface acoustic waves beside the IDTs. Namely, the SAW device may provide a resonator function. Since the SAW device can delay propagation speed to about 10.sup.-5 times the speed of an electromagnetic wave, it may serve as a delay device.
These functions of the SAW devices are applicable for realizing filters, resonators, delay lines, etc., which are compact, inexpensive, and adjustment free. For example, they are used for intermediate frequency (IF) filters, resonators of oscillators, voltage control oscillators (VCOs), etc. These days, the SAW devices are operated at higher frequencies, and because they are compact and inexpensive, they are also used for band-pass filters for mobile radio equipment such as automobile and portable telephone equipment. Therefore, the SAW devices are required to have a small insertion loss and predetermined pass-band characteristics over a broad bandwidth.